Method of anodic oxidation of aluminum



United States Patent 3,445,349 METHOD OF ANODIC OXIDATION OF ALUMINUM Sakae Tajima, Tokyo, Nobuyoshi Baba, Tokorozawa, Saitama, Shin Matsuyama, Toyokawa, Aichi, and Masabumi Ono, Koda, Nukata, Aichi, Japan, assignors to Minoltacamera Kabushiki Kaisha, Osaka, Japan No Drawing. Filed Nov. 30, 1966, Ser. No. 597,850

Int. Cl. C23b 9/02 US. Cl. 20414 Claims This invention relates generally to an electrolyte composition and a process for anodic oxidation for aluminum. More particularly, the present invention relates to a novel electrolytic solution and a process for the anodic oxidation of aluminum to provide a colored, strongly adhered oxidation layer on the surface of the aluminum-containing metal.

Electrolytic surface oxidation of metals is known in the art for purposes of improved appearance or pdurability. With respect to the oxidation of aluminum, the known process includes the positioning of aluminum work piece that is to be oxidized as an anode in an electrolytic solution. When the voltage is impressed between the anode containing the aluminum work piece and a cathode positioned in the electrolyte, an oxidized aluminum layer or coating is formed on the anode work piece. The oxidized aluminum surface obtained in the past has not been found to be satisfactory, since the layer possessed a porosity which is too high to adequately resist corrosion. Further, and also of significant importance, it has been found that the oxidized layer must undergo a separate and distant dye treatment, in order to achieve a desired coloration. This added dye step obviously resulted in a more costly and therefore less economically feasible process.

There has been great effort expended in the direction of improving the oxidized layer formed for anodic oxidation (see Japanese Patent No. 442,536, of Tajima and Baba, among the present inventors, who discovered a process for anodic oxidation utilizing an electrolyte solution containing formamide and boric acid). This electrolyte eliminated many of the problems facing the art in that it made it possible to increase the thickness of the oxidized coating formed on the aluminum surface through the selection of the time and current density. At the same time this process produced a change in color of the oxidized coating varying from a yellow to a black. While it initially seemed that the ultimate goals of the art were attained by this discovery, it was found that such electrolyte was deficient in that a continuous production of articles was not possible. Also, the coloring was found to vary frequently in successive articles which passed through the electrolyte of that anodizing process.

The electrolyte, though initially adequate, deteriorated rapidly, particularly under high voltage, and had a useful life considerably shorter than desired. Lowering the voltage from the peak final voltage thought to be required as one way of extending the useful life of the electrolyte necessarily resulted in prolonging the production or deposition time per unit thickness to a point beyond which the entire process became economically unsound.

Accordingly, the primary object of the present invention is the provision of an electrolyte composition and process which, through the permissible use of a lower voltage, does not either undesirably decrease the thickness of the oxidation layer on the surface of the aluminum or the time required.

A further object of the present invention is the provision of a process and an electrolyte which produces in a single step an oxidation layer which possesses a low porosity, high corrosion resistance, a high degree of resistance against wear, does not develop cracks or crevices even 3,445,349 Patented May 20, 1969 with increased thickness of the coating, and attains a color variable in accordance with the thickness of the layer.

Briefly, the present invention includes a process for the anodic oxidation of aluminum which permits a lower impressed voltage to be used while retaining a per unit thickness of the oxide coating per unit time and includes also a novel composition containing formamide in an amount that may be between 50% to 80% by weight, boric acid in an amount that may be 16% to 40% by weight, and an amount of oxalic acid anhydride which is present in an amount suificient to permit the reduction of the impressed voltage while retaining per unit thickness of the oxide coating per unit time and which oxalic acid anhydride may be present in an amount between 1% and 15% by weight of the electrolyte.

The electrolyte composition of the present invention comprises a synergistic mixture of formamide, boric acid, and oxalic acid anhydride.

Formamide is a liquid and as such is used to dissolve the boric acid and oxalic acid anhydride. The weight of the formamide may vary considerably, but preferably is within the range of 50% to 80% by weight of the electrolyte. More particularly the formamide is desirably present in an amount between 60% and The boric acid also may vary considerably and may be between 16% and 40% by weight of the electrolyte composition, and generally is about 30% to 50% of the weight of the formamide. Preferably, the boric acid is in an amount between 20% and 35% of the total electrolyte composition.

The oxalic acid anhydride is an essential ingredient in the electrolyte composition of the present invention and may vary between 1% and 15% by weight of the electrolyte, and more particularly between approximately 4% and 8% by weight of the electrolyte. Within this range a composition of 5% to 6% has been found to be unexpectedly unique in the results produced. Based upon the weight of the formamide and boric acid, it may be stated that the oxalic acid anhydride is approximately 2% by weight of the two greater components of the tri-component electrolyte of the present invention.

The aluminum to be oxidized can be solid aluminum or any aluminum alloy and is positioned as an anode. The cathode is also composed of aluminum and may be in the form of aluminum plate.

The process for the anodic oxidation of the aluminum according to the present invention includes the electrolyte, as set forth, and the aluminum work piece which is to be anodized, along with the aluminum cathode placed in an electrolytic cell. Initially, when the aluminum anode does not possess a coating thickness, a considerably lower voltage may be utilized to pass current between the cathode and anode. However, as the thickness of the anodized coating builds up on the anode, the electric resistance of the anode also increases, thus requiring a gradually increased voltage to be impressed up to a maximum usually referred to as the final voltage, in order to maintain a constant current density and a uniform deposition. The purpose of maintaining the current constant at any par ticular level is merely to permit the results to be more easily reproduced in successive work pieces. It should be understood, however, that a particular voltage or current density or a constant density is not an essential requirement of the present invention.

The color of the oxidation layer which develops changes in accordance with the thickness of the coating and generally forms a yellow to a black and will be blacker the thicker the layer. To attain the desired color in accordance with the present invention, a selected combination of current density and time should be made.

A particular selection of current density and time is not an essential aspect of the present invention.

The following example will illustrate the process of the present invention:

EXAMPLE I Added quantity of oxalic acid anhydride (weight, percent) Surface to be treated (cm?) Final voltage (v.)

It will be noted that the final voltage required to maintain the current density constant drops considerably from the final voltage required in the absence of oxalic acid anhydride. It should also be noted that when the quantity of oxalic acid anhydride is 5% to 6%, the amount of surface that can be treated is substantially higher before the deterioration of the electrolyte. At this approximate range of concentration, it is to be noted that the final voltage is almost one-half as low as the final voltage required without the presence of oxalic acid anhydride. The life span of the electrolyte is increased two to three times over the working life of the electrolyte without the oxalic acid anhydride present.

Further, the coating obtained in accordance with the foregoing examples in the range of 5% to 6% oxalic acid anhydride have a black-brownish oxidized coating of a thickness of approximately 32 and a diamond pyramid hardness measured as a Vickers of 473. As a further test to the integrity of the anodized coating, the anodized aluminum surface was sand blasted for 30 minutes, in accordance with the Japanese Industrial Standards Test without any notice of deterioration.

EXAMPLE II An electrolytic solution of 66.6% by weight formamide, 33.3% boric acid, and 5.% oxalic acid anhydride is put into an electrolytic cell. Into this cell are put an aluminum plate with the surface area of x 7 cm. as an anode and another aluminum plate of the same shape with the same surface area as a cathode. Anodic oxidation is effected under the following conditions:

Current density A./dm. 2 Solution temperature C 55 i5 Time min 45 Final voltage v 130 After the foregoing anodie oxidation, a dark brown coating of 32p in thickness with the Viekers hardness of 473 is produced. The properties of the coating are as mentioned in the specification, The surface is smooth without any minute cracks and is highly anti-corrosive.

It has been shown, therefore, that in accordance with the present invention the final voltage of the anodizingprocess can be lowered and at the same time prolong the life span of the electrolyte through the addition of a sufiicient amount of oxalic acid anhydride, and that a color strongly adhering oxidation coating can be obtained on the aluminum surface, which is color-fast to light and withstands severe abrasive tests.

From the foregoing detailed description it will be evident that there a number of changes, adaptations, and modifications of the present invention which come within the province of those skilled in the art. However, his intended that all such variations not departing from the spirit of the invention be considered as Within the scope thereof as limited solely by the appended claims.

We claim:

1. An electrolytic composition consisting essentially of:

50 %-80% by weight formamide;

16%-40% by weight boric acid, and

1%15% by weight oxalic acid anhydride.

2. The electrolytic composition of claim 1, wherein formamide is present in the amount of 60% to boric acid is present in the amount of 25% to 35%, and oxalic acid anhydride is present in the amount of 4% to 8%.

3. In an electrolytic composition for producing a coating of oxide on aluminum forming the anode for anodic oxidation consisting essentially of formamide and boric acid, the improvement comprising the addition of oxalic acid anhydride in an amount suflicient to effect a reduction in the voltage requirement per unit thickness of oxide coating per unit of time.

4. The composition of claim 3, wherein the oxalic acid anhydride is present in the amount of 4% to 8%.

5. The process comprising forming an oxidation coating on an aluminum surface by anodic oxidation in the presence of an electrolyte comprising formamide, boric acid, and an amount of oxalic acid anhydride sufiicient to lower the required impressed voltage while retaining per unit thickness of the oxide coating per unit time.

6. The process of claim 5, including providing an amount of oxalic acid anhydride between 1% to 15% by weight of the electrolyte.

7. The process of claim 6, wherein the formamide is present in the amount of 50% to by weight and the boric acid is present in the amount of 16% to 40% by weight.

8. The process of claim 7, wherein the amount of oxalic anhydride is between 4% to 8% by weight.

9. The process of claim 7, wherein the final impressed I References Cited UNITED STATES PATENTS 8/1938 Edwards 204-58 4/1967 Wales 204-l4 HOWARD S. WILLLAMS, Primary Examiner.

T. TUFARIELLO, Assistant Examiner. 

1. AN ELECTROLYTIC COMPOSITION CONSISTING ESSENTIALLY OF: 50%-80% BY WEIGHT FORMAMIDE; 16%-40% BY WEIGHT BORIC ACID, AND 1%-15% BY WEIGHT OXALIC ACID ANHYDRIDE. 